But the rising availability of plant-based alternatives, and increasing concerns about seafood’s sustainability and carbon footprint, have led some to question whether we need it in our diets. Since 1974, the UN's Food and Agricultural Organization reports, fish stocks within biologically sustainable levels have declined from 90% to just under 66% today.

Meanwhile, concerns over mercury and other pollutants mean women who are pregnant or breastfeeding, for example, are told to limit their consumption of some species.

Does eating fish provide more health benefits – or health risks?

Heavy metal

In recent decades, one of the biggest concerns about fish has been its potentially harmful levels of pollutants and metals.

One concern is polychlorinated biphenyls (PCBs). Although they were banned by the 1980s, these industrial chemicals were used worldwide in huge quantities and still linger in our soil and our water. They’ve been associated with a range of negative health effects on everything from the immune system to the brain. While PCBs are present in everything from dairy products to drinking water, the highest levels tend to be found in fish.

The solution for limiting your intake of PCBs from fish may be counterintuitive, says Johnathan Napier, science director at Rothamsted Research in Hertfordshire, England.

“The possible problem of the accumulation of toxic compounds is likely to be more of concern for wild species that are caught for direct human consumption,” he says. Because the marine-derived ingredients that farmed fish are fed are cleaned or scrubbed to remove toxins, farmed fish is often safer than wild.

That isn't always the case, however, and PCB content also fluctuates seasonally.

While they are generally viewed as better for our health and the environment, large-scale aquaculture has its own problems, such as polluting the oceans with waste and becoming breeding grounds for diseases that can spill over into the wild.

The NHS recommends that pregnant and breastfeeding women limit their intake of fish species more likely to contain PCBs, as well as other pollutants like dioxins, to two portions per week. These fish include oily fish like salmon and sardines, as well as non-oily fish including crab and sea bass. A portion is around 140g.

Another worry is mercury, a neurotoxin that could pass through the placenta and affect child development. There are numerous links between mercury ingestion and cancer, diabetes and heart disease. While mercury can be found in other foods, such as vegetables, one study found that 78% of participants' mercury intake came from fish and seafood.

In fish, mercury levels are high enough for the US Food and Drug Administration (FDA) to recommend that pregnant people limit their intake of some popular fish, including halibut and tuna, to one serving a week.

But concerns around the accumulation of heavy metals in fish has been overexaggerated, says Napier. He says it’s only a problem when it comes to species that live a particularly long time – like swordfish, which can live for 15 to 20 years. Swordfish has a mercury concentration of 0.995 PPM, while salmon, which lives on average for four to five years, has around 0.014. While research is still ongoing, the US's Environmental Protection Agency currently states that for pregnant women, the highest allowable average mercury concentration per serving, if eating one serving a week, is 0.46 PPM.

But the issue is set to worsen, as there’s evidence to suggest that levels of mercury found in the ocean may rise as the planet warms. Research has found that as Arctic permafrost melts, it releases mercury that was trapped in frozen ground into waterways.

While mercury poses a small risk, Napier says there stands to be much more to gain from fish – particularly marine omega 3.

Fatty acids

Consumption of oily fish, including salmon, tuna, sardines and mackerel, has been linked to a lower risk of cardiovascular disease, thanks to its marine omega 3 fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA).

Some plant-based sources of omega 3, such as flax seeds and walnuts, are rich in a third type – ALA. A 2014 study concludes that the heart health benefits of plant-based omega 3 may be comparable to EPA and DHA, but there isn't research to back it up yet. However, you can find both EPA and DHA in algae supplements and in edible seaweed.

DHA is abundant in our brains, retinas and other specialised tissues. Along with EPA, it helps to fight off inflammation in the body, which is linked to higher risk of heart disease, cancer and diabetes.

“Population data looking at the effects of marine omega 3 on health is consistent and strong, and shows that people with a higher intake of EPA and DHA have a lower risk of developing common diseases, particularly heart disease, and dying from them,” says Philip Calder, head of human development and health at England’s University of Southampton.

One way to avoid potential damage from mercury exposure while still getting omega 3 is to take fish oil supplements. However, research recently carried out on behalf of the World Health Organization (WHO) looking at omega-3 supplements across range of health outcomes found they don’t have the same effect as eating oily fish.

“Our bodies are adapted to metabolising whole foods, rather than a single slug of a particular nutrient or ingredient,” Napier says.

“Our findings suggest a very small beneficial effect [in terms of lowering the risk] of dying of coronary heart disease,” adds Lee Hooper, a reader at the University East Anglia and one of the WHO study’s researchers.

Around 334 people would have to take omega-3 supplements for four or five years for one person not to die from coronary heart disease, she says.

But there’s an issue with population studies like Hooper’s. While some oily fish, such as sardines, aren’t relatively expensive, fish is generally associated with a more expensive diet. It’s widely accepted that socioeconomic status affects health outcomes – so it’s possible that families who eat more fish also have higher incomes and healthier lifestyles in general.

Normally, researchers will take into account such confounding factors, Calder says, but they might not think of everything that could skew a study’s results. The WHO report was a review of 79 studies, which each will have differed in how they controlled for participants' socioeconomic status.

But intervention trials, where people are randomly assigned to a group and an intervention such as taking omega-3 supplements is measured, have problems, too. Analysing potential health impacts of EPA and DHA deficiency, for example, is difficult, Calder says, because people start trials with varying levels of omega-3 in their systems.

In addition, research shows that fish might impact everyone’s health to varying degrees, depending on how well they can convert precursor forms of EPA and DHA. This difference could come down to a person’s overall diet and lifestyle, Calder says, but genetic differences could also play a role.

Another reason the health benefits of fish may vary is because of how fish are raised.

Marine ecosystems are full of omega-3: little fish eat marine plankton, and get eaten by bigger fish, and the whole food chain passes on omega-3 to humans. But the system is different for farmed fish, which is what most of us eat. “In a fish farm, it’s just thousands of fish in a cage. They eat what they’re given by the fish farmer,” Napier says.

As they would in the wild, farmed fish normally are fed smaller fish species. In the wild, however, fish would eat a variety of smaller fish. In farms, fish are often fed fish meal made from Peruvian anchovies.

But these anchovies are already being fished at the maximum level at which the industry can be sustained, Napier says – even as global aquaculture is expected to keep growing. According to the UN’s Food and Agricultural Organization, growing demand for fish oil supplements means that the fish oil contained in the fish meal fed to farmed fish is diminishing. That means the amount of omega-3 in the fish we consume is declining, too.

“There are finite levels of omega fish oils that come out of the ocean each year – that’s all we’ve got,” he says. “If aquaculture is expanding but the most important input you need to put into people’s diets, the fish oil, is completely static, you’re diluting how much is fed to the fish.”

Research from 2016 found that levels of EPA and DHA in farmed salmon decreased by half over a decade. Even so farmed salmon still has more omega-3 than wild salmon, Napier says.

“Wild salmon swims back forth across the Atlantic; it’s a lean animal. It’s not laying down fat because it’s burning everything it consumes,” he says.

Brain food

Aside from omega-3, fish has other beneficial nutrients, including selenium, which protects cells from damage and infection; iodine, which supports a healthy metabolism; and protein.

Fish has long been hailed as “brain food”. A recent study suggests this isn’t just thanks to its omega-3 content – although studies have also found a link between omega-3 and slower cognitive decline.

Researchers compared brain volumes in people who consumed fish with those who didn’t, and found that baked or broiled fish is associated with larger grey matter volumes, independent of omega-3 levels.

“Our brain volumes change with improved health and disease. The more neurons you have, more brain volume you have,” says Cyrus Raji, assistant professor of radiology and neurology at Washington University School of Medicine.

Researchers compared the fish-eating habits and MRI scans of 163 participants who were in their late 70s, on average. They found that, compared to participants who didn’t eat any fish, those who ate fish on a weekly basis had larger brain volumes – mostly in their frontal lobe, which is important for focus, and in their temporal lobes, crucial for memory, learning and cognition.

The relationship between fish and the brain could be down to fish having an anti-inflammatory effect, Raji says, because when the brain responds to reduce inflammation, it can affect brain cells in the process.

“This means you can improve brain health and prevent Alzheimer’s with something as simple as the dietary consumption of fish,” says Raji. To make the brain as resilient as possible to dementia, Raji advises starting to eat fish at least once a week when you’re in your twenties or thirties.

Another reason fish can be healthy is because it replaces less healthy foods in our diets. “If we eat more fish, we tend to eat less of other things,” says Hooper.

Still, because there isn’t robust research suggesting major health inadequacies for people who don’t eat fish, Calder says it’s difficult to definitively say that fish is essential to overall human health. However, he adds, it is clear that omega-3 promotes health and reduces the risk of disease.

But getting to the bottom of how healthy fish really is may be a moot point after long. “Since fish isn’t a sustainable food source, research now will probably focus on solutions to that – such as how to grow algae and harvest omega-3 oil, instead of more studies into fish,” Calder says.

Individuals can help by choosing the most sustainable fish species available. Guides like the one by the Marine Conservation Society show which fish are the best, with 50 of the 133 species listed coming up as mostly sustainable, “good” choices – including, fortunately, popular favourites like farmed salmon, prawns, cod, mackerel, mussels, oysters and farmed halibut.

BBC

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